The present invention relates a brake-by-wire type brake system for a vehicle, which system is arranged to generate braking force requested by a driver's brake manipulation using a unit hydraulic pressure obtained by controlling a hydraulic pressure of a hydraulic source different from a brake hydraulic source directly operated by the driver's brake manipulation.
Various electronically-controlled hydraulic brake systems have been proposed and put in practical use, in order to meet the need of executing a vehicle behavior dynamic control using an automatic braking and/or a right-and-left wheel braking force difference control and the need of electronizing an anti-skid control system. For example, Japanese Published Patent Application No. 2000-168536 discloses an electronically-controlled brake system in which an electronically-controlled shutoff valve is disposed in a brake hydraulic pressure circuit for supplying a hydraulic pressure to wheel cylinders from a master cylinder operating in response to the depression of a brake pedal. The electronically controlled brake system further comprises a hydraulic source constructed by a pump for receiving working fluid of a reservoir of a master cylinder and discharging the working fluid, an electric motor for driving the pump and an accumulator for accumulating the working fluid supplied from the pump. This brake system executes an electronic control of a brake-unit hydraulic pressure independently of the control of a master cylinder hydraulic pressure, by increasing the brake-unit hydraulic pressure in the wheel cylinder using the accumulator inner pressure through a pressure increasing valve and decreasing the brake-unit hydraulic pressure through a pressure decreasing valve under a condition that the brake hydraulic pressure circuit of connecting the master cylinder and the wheel cylinders is shut off.